/Engine/Source/Runtime/Experimental/GeometryCollectionEngine/Public/GeometryCollection/GeometryCollectionISMPoolComponent.h
Add a group to the ISM. Returns the group index.
[FInstanceGroups::FInstanceGroupId](API\Runtime\GeometryCollectionEngine\GeometryCollection\FInstanceGroups\FInstanceGroupId) AddInstanceGroup ( int32 InstanceCount, [TArrayView](API\Runtime\Core\Containers\TArrayView)< const float > CustomDataFloats )